Method for sealing and/or joining an end of a ceramic filter

ABSTRACT

A process for sealing a ceramic filter by infiltrating a metal into an end of the filter. The process includes the steps of contacting the end of a porous ceramic filter with a molten metal, whereby the metal enters into the ceramic matrix to substantially fill the void space. The ceramic filter is cooled to form a filter having a ceramic-metal composite portion. The present invention also provides a filter having an infiltrated metal seal. Methods for joining infiltrated ends are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/220,558, filed Mar. 31, 1994, now U.S. Pat. No. 5,700,373,issued on Dec. 23, 1997, which is a continuation-in-part of PatentCooperation Treaty Application No. PCT/US93/08835, filed Sep. 17, 1993,which designates the United States and is a continuation-in-part of U.S.patent application Ser. No. 07/946,972, filed Sep. 17, 1992, nowabandoned.

FIELD OF THE INVENTION

The present invention relates to a method for sealing and/or joining anend of a porous ceramic, preferably a ceramic filter element. Moreparticularly, the present invention relates to a method for sealingand/or joining the end of a ceramic filter element by infiltrating metalinto a portion of the filter element. The present invention also relatesto filter elements produced by infiltrating metal into an end to sealthe filter element, and to connecting devices for connecting such filterelements to a dissimilar material or together.

BACKGROUND OF THE INVENTION

Filtration devices are used to separate two or more substances from eachother on the basis of chemical or physical properties of the substances.Filtration devices can rely upon a chemical potential differentialacross a porous membrane for separating the substances. There has beensignificant interest in a range of filtration devices that can becategorized into groups including: (1) dense membranes in whichmaterials diffuse through grain boundaries; (2) micropores (from about 3to about 20 angstroms); (3) nano-filtration (from about 10 to about 80angstroms); (4) ultra-filtration (from about 0.001 to about 0.2micrometers); and micro-filtration (from about 0.2 to about 10.0micrometers).

These filtration devices are particularly useful for separatingsubstances from gas streams or liquids. A number of separation methodshave been used in the various processing industries. The use offiltration devices having ceramic membranes is a relatively new area.The benefits of ceramic membranes generally include high-temperaturecapability, resistance to chemicals and good structural integrity whichpermits the separation membrane to be used under high pressures.

Ceramic membranes can be used for a wide variety of applications. Forexample, clarifying and sterilizing fruit juices and other liquids inthe food and beverage industry; concentrating vaccines and enzymes orpurifying amino acids and similar processes in the biotechnologyindustry; removing hydrogen from refinery streams and carbon dioxide andhydrogen sulfide from natural gas in the gas separation industry;separating oxygen from air; removing precipitated radionuclides andmetal oxide and metal hydroxide particles from waste water; andpurifying waters, acids, solvents and similar liquids in the electronicmanufacturing industry.

Ceramic-based filter elements have been developed to take advantage ofthe properties of ceramic materials. For example, a filtration devicehaving a ceramic filter element is disclosed in U.S. Pat. No. 4,069,157by Hoover et al., which is incorporated herein by reference in itsentirety. This patent discloses a filter element fabricated using aporous ceramic support, such as alumina (A1 ₂O₃) or cordierite(2MgO·2Al₂O₃·5SiO₂), having a porosity of from about 30 percent to about60 percent. A ceramic membrane layer is coated onto the interiorchannels of the porous ceramic support. The opening size in the membraneis controlled and can vary from about 0.002 micrometers up to about 1micrometer.

Similar filtration devices utilizing ceramic filter elements aredisclosed in the prior art. For example, see U.S. Pat. Nos. 4,894,160and 4,971,696, both by Abe et al.; U.S. Pat. No. 4,983,423 by Goldsmith;or U.S. Pat. No. 4,981,590 by Van Tveen. Each of the foregoing patentsis incorporated herein by reference in their entirety.

One of the problems associated with manufacturing filtration devicesincorporating ceramic filter elements is the difficulties andlimitations relating to sealing the end of the ceramic filter element.At least one end of the filter element must usually be sealed andprepared for installation into a filtration device by sealing the poroussupport and providing a surface that can easily be attached to thefiltration device. Typically, the end of the filter must form a tightseal with a metal component, such as a stainless steel ring, to preventthe pressurized filtrant from bypassing the filter. Most filter elementshave been sealed by internally sealing the porous ceramic with a ceramicslurry or cement and using organic materials, such as rubber or polymer“o-rings” to seal around the perimeter of the filter element. Theo-rings are typically not capable of functioning in elevatedtemperatures and under corrosive conditions. Further, the o-rings willnot form a tight seal around the ceramic if the perimeter of the ceramicfilter element has defects or is “out of round.” This is a particularlyacute problem when the filter is used to separate materials having verysmall diameters (e.g. ultrafiltration).

U.S. Pat. No. 5,203,488 by Wang et al. issued on Apr. 20, 1993. Thispatent is assigned to LANXIDE Technology Co. and is part of a series ofpatents assigned to LANXIDE Technology Co. that relate to ceramic-metalcomposites. Wang et al. disclose a method for joining twoself-supporting bodies by a reactive infiltration process. It isdisclosed that two materials can be bonded together utilizing thecomposite of the invention. For example, a powdered parent metal and amaterial which is to be reactively infiltrated can be placed between thetwo bodies. In an alternative embodiment, it is disclosed that an activebrazing material can be placed between two composites formed accordingto the invention or may be placed between one body formed in accordancewith the invention and a second body. It is disclosed that a foil, pasteor powder which includes an active brazing alloy is placed between atleast two self-supporting bodies made according to the first step of theinvention.

SUMMARY OF THE INVENTION

The present invention is generally directed to a method for sealingand/or joining an end of a porous ceramic and preferably a ceramic-basedfilter element.

According to one aspect of the present invention, a method for sealingthe end of a ceramic filter element is provided. The method can includethe steps of providing a porous ceramic filter element having a firstend and a second end and having filtering channels therethrough,contacting a portion of the first end of the filter element with amolten metal to infiltrate the metal into the first end of the filterelement and cooling the infiltrated portion to form a filter elementhaving a sealed end comprising a ceramic-metal composite portion.

According to certain embodiments of this aspect of the invention, themethod can include the step of attaching a metal seal ring to theceramic-metal composite portion. The step of attaching a metal seal ringto the ceramic-metal composite portion can include the step of brazingor welding a seal ring to the ceramic-metal composite portion. Accordingto another embodiment, the ceramic filter element can include porousalumina having an open porosity of from about 30 volume percent to about50 volume percent and the metal can include copper metal. The coppermetal can also include an infiltration additive, such as oxygen. Whenoxygen is used as an infiltration additive, it is preferably added in anamount from about 1.5 weight percent to about 10 weight percent. Theporous ceramic support can have an average pore size of from about 0.01micrometers to about 2 millimeters, preferably from about 2 micrometersto about 15 micrometers, more preferably from about 6 micrometers toabout 12 micrometers.

In another embodiment of this aspect of the invention, the method canfurther include the step of attaching a connecting means to theceramic-metal composite portion. In one embodiment, such a connectingmeans permits sealable attachment of the porous ceramic element to adissimilar material. In another embodiment, such a connecting meanspermits sealable attachment of one porous ceramic element to anotherporous ceramic element. In further embodiments, such a connecting meanscan include a swage fitting, a compression fitting, a weld, a braze, ametal bellows and/or threads which are formed on the ceramic metalcomposite portion.

According to another aspect of the present invention, a method forsealing the end of a ceramic-based filter element is provided. Themethod can include the steps of placing a metal in a refractory vessel,heating the metal to a temperature in excess of the melting temperatureof the metal such that the metal is in the form of a molten pool havinga top surface, contacting a cylindrical ceramic filter element having afirst end and a second end and a plurality of channels therethrough withthe top surface of the molten pool for a time sufficient to infiltrate aportion of the sintered ceramic filter element, removing the sinteredceramic filter element from contact with the molten pool, cooling themetal-infiltrated portion to form a ceramic filter element having aceramic-metal composite portion and attaching a metal seal ring to theceramic-metal composite portion.

According to certain embodiments of this aspect of the invention, theattaching step can include the step of brazing a seal ring to aceramic-metal composite with a brazing alloy. The brazing alloy caninclude a copper-silver alloy. In one embodiment, the metal seal ring isa stainless steel ring. In other embodiments, the metal seal ring can bemade from other metals, including but not limited to, Kovar™ and carbonsteel.

According to another aspect of the present invention, a method forsealing the end of a porous ceramic filter element is provided. Thismethod can include the steps of providing a porous ceramic filterelement having a first end and a second end and comprising a pluralityof channels therethrough, providing a metal seal ring located around theperimeter of the first end of the filter element, contacting the firstend of the filter element with a molten reactive braze metal toinfiltrate a portion of the first end with the reactive braze metal andcooling the reactive braze metal to form a seal between the filterelement and the seal ring. Preferably, the reactive braze metal isselected from the group comprising titanium, copper, nickel, silver andalloys thereof.

In yet another aspect of the present invention, a ceramic filter elementfor a filtration device is provided. The ceramic filter element includesa substantially cylindrical ceramic porous support having channelstherethrough, a membrane layer coated on at least a portion of saidchannels in said porous ceramic support and a metal infiltrated into aportion of the cylindrical ceramic filter element to form aceramic-metal composite therein.

In another embodiment of the present invention, the ceramic filterelement has a tubular configuration. The tubular configuration is madeup of a central channel surrounded by a porous sidewall. The substanceto be filtered can be passed adjacent either the inner or outer wall ofthe tubular porous sidewall and the filtered material passes through theporous sidewall to the other side for collection.

According to certain embodiments of this aspect of the invention, thefilter element includes alumina ceramic. The filter element can have anopen porosity of from about 30 volume percent to about 50 volumepercent. Further, the filter element can include a stainless steel ringattached to the ceramic-metal composite portion.

In other embodiments of this aspect of the invention, the filter elementfurther includes a connecting means that is attached to theceramic-metal composite portion. In further embodiments, such aconnecting means can include a swage fitting, a compression fitting, aweld, a braze, a metal bellows and/or threads which are formed on themetal composite portion for screwing the ceramic filter element into athreaded member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrates a ceramic filter element according to oneembodiment of the present invention.

FIG. 2 illustrates a ceramic filter element according to one embodimentof the present invention.

FIG. 3 illustrates a ceramic filter element according to one embodimentof the present invention that has been infiltrated with a metal to sealan end of the filter.

FIG. 4 illustrates a filter element according to one embodiment of thepresent invention that has been infiltrated with a metal to seal an endof the filter and has a metal ring brazed to the sealed end.

FIG. 5A illustrates a plurality of ceramic filter elements according toone embodiment of the present invention that are connected to a tubesheet header.

FIG. 5B illustrates a cylindrical housing that contains a plurality ofceramic filter elements according to one embodiment of the presentinvention which are connected to a tube sheet header.

FIG. 6 illustrates a joint between two ceramic filter elements that isformed by a thread connection according to one embodiment of the presentinvention.

FIG. 7 illustrates a joint between two ceramic filter elements having aswage fitting connection according to one embodiment of the presentinvention.

FIG. 8 illustrates a plurality of ceramic filter elements that areattached to a tube sheet header by a brazed ring connection means thathas been flared into the header to form a wedge seal according to oneembodiment of the present invention.

FIG. 9A illustrates two ceramic filter elements of the present inventionwhich are attached by a butt joint according to one embodiment of thepresent invention.

FIG. 9B illustrates a tapered fit lap joint between two ceramic filterelements of the present invention.

FIG. 10A illustrates two ceramic filter elements of the presentinvention that are joined by a metal bellows connection according to oneembodiment of the present invention.

FIG. 10B illustrates the use of metal bellows to connect ceramic filterelements of the present invention to a tube sheet header.

FIG. 10C illustrates the use of metal bellows to connect ceramic filterelements of the present invention to each other end to end.

FIG. 11 illustrates a ceramic filter element that is sealed at one endwith a metal cap according to one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is directed to a method for sealing the end of aporous ceramic element, preferably ceramic filter element, byinfiltrating a metal into the end of the filter element to form aceramic-metal composite at the end. The composite at the end of thefilter element creates a substantially hermetic seal within the poroussupport and a metal ring can be attached to the composite in preparationfor placement into a filtration device.

Ceramic filter elements typically have a cylindrical configuration, suchas that illustrated in FIG. 1, although other configurations arepossible. Referring to FIG. 1, a cross-flow filter element 110 isillustrated as a cylinder including a plurality of channels 120 that runthrough the length of the filter element 110 and are substantiallyparallel to the cylinder and one another. The filter element can be openat both ends or one end can be closed. During use, the substance to befiltered passes into a first (upstream) end 130 of the filter element110 and a portion of the filtrate passes through the sidewalls 140 ofthe filter element 110 due to a pressure differential.

As is illustrated by the cut-away portion of FIG. 1, the channels 120are preferably coated on their interior surface 150 with a membrane.Thus, before the filtrate reaches the porous support 160 of the filterelement 110, the filtrate must pass through the membrane layer coated onthe walls of 150 of the channels 120.

The ceramic porous support can be chosen from any of a number of metaloxides, carbides, nitrides or the like. For instance, the ceramic cancomprise alumina (Al₂O₃), cordierite (2MgO·2Al₂O₃·5SiO₂), titania(TiO₂), zinc oxide (ZnO), zirconia (ZrO₂), iron oxide (Fe₂O₃), magnesia(MgO), silica (SiO₂), or any other metal oxide. Further, non-oxideceramics such as silicon carbide (SiC), silicon nitride (Si₃N₄),aluminum nitride (AlN) or titanium diboride (TiB₂) can be used as theceramic support material. According to one embodiment of the presentinvention, alumina is preferred since porous alumina supports can beeasily formed and processed into various shapes. Further, aluminamaintains good strength and structural integrity in a variety ofconditions.

The porous support of the ceramic filter element is a sintered, coherentceramic body that has substantially continuous open porosity. The poroussupport preferably has an apparent open porosity of from about 10 volumepercent to about 90 volume percent, more preferably from about 30 volumepercent to about 50 volume percent. The open porosity preferably has anaverage apparent diameter of from about 0.01 micrometers to about 2millimeters, preferably from about 2 micrometers to about 15micrometers, more preferably from about 6 micrometers to about 12micrometers.

The porous support can be formed in a number of ways known to thoseskilled in the art. For example, porous ceramics can be formed byincorporating pore formers into the ceramic batch. Pore formers arematerials that have a volatilization temperature that is lower than thesintering temperature of the ceramic. Upon sintering, the materialvolatilizes, leaving behind open pore space. Examples of such materialsinclude walnut shells and latex spheres.

Porous ceramics can also be produced by controlling the startingparticle size of the ceramic raw materials, the sintering aids and thesintering temperature used to densify the ceramic. For example, porousalumina can be formed by blending alumina particles having particle sizeof about 10 micrometers. The particles are formed into a cohesiveplastic mass and extruded through a die to form a green body havingchannels therethrough. The green body is then sintered for a period oftime to form a sintered ceramic porous support. The sinteringtemperature is somewhat below the normal sintering temperature forobtaining dense alumina so that a porous structure can be obtained.

The porous support has at least one channel running through the interiorof the filter and preferably has a plurality of channels. The channelscan be formed by a number of techniques known to those skilled in theart, such as incorporating temporary channel molds that are removableafter sintering or forming the channels during extrusion of the ceramicbody out of an extrusion die.

After sintering, the ceramic porous support can then be treated toadhere one or more membrane layers to the channels of the porous supportto form a filter element. The membrane layers are the primary filteringlayers for separation of materials. A membrane layer typically includesa dispersion of particles having a controlled particle size and particlesize distribution. The particles can be dispersed in a liquid mediumwhich is slip cast, dip-coated or spray-coated onto the channels of theporous ceramic support. The membrane particles can have a similarceramic composition as the porous support or can have a differentcomposition. It is preferred, however, that the membrane layer be madefrom ceramic materials to maintain corrosion and heat resistance. Theporosity and pore size of the membrane is controlled by the size of themembrane particles and the temperature and time of sintering after theporous support is coated with the membrane.

The membrane layer can include a plurality of layers having differentparticle sizes for different applications. Each individual layer can beapplied, for example, by immersing the tube in a liquid suspension anddrawing a vacuum through the inner diameter of the tube. Alternatively,the slurry containing the membrane particles can be pumped through thetube under pressure or can be slip-cast into the tube.

According to one preferred embodiment of the present invention, themembrane includes three different layers. A first layer is applied tothe channels by slip-casting a slurry having alumina particles with anaverage diameter of about 3 micrometers and this layer is sintered atabout 1430° C. to form an adhered first layer having an average poresize of about 0.8 micrometers. A second layer is then applied byslip-casting alumina particles onto the first layer having an averageparticle size of about 0.5 micrometers. This layer is sintered at about1000° C. to form a second adhered layer having an average pore size ofabout 0.2 micrometers. Finally, a third layer can be applied as asol-gel and can be sintered at about 650° C. to form a third layerhaving an average pore size of, for example, about 40 angstroms.

According to the present invention, when a filter element having thedesired properties is obtained, molten metal can be infiltrated into theopen porosity of the filter element to internally seal the filterelement. The metal infiltrates the porous support without substantiallyclogging or filling the filter channels. The metal can be infiltratedinto the ceramic filter element either before or after the membranelayer has been adhered to the porous support. It may be advantageous toinfiltrate the metal into the filter element before applying themembrane layers since the molten metal may preferentially infiltrate themembrane layer due to the smaller pore size of the porosity in themembrane layer and because the infiltration temperature can be hotterthan the sintering temperature of the membrane layer.

The infiltrant metal can be selected from any metal whose melting pointis below the melting point of the ceramic matrix material. For example,copper (Cu), nickel (Ni), aluminum (Al), silver (Ag) or alloys thereofcan be used for the infiltrating metal. A preferred metal is copper,since it has been found that copper alloy readily infiltrates into analumina ceramic filter element. Other metals may be preferred, dependingon the anticipated end use of the filter.

In a preferred embodiment of the present invention, an end of theceramic filter element is brought into contact with the molten metal andthe metal infiltrates the ceramic by capillary action without theassistance of any substantial overpressure. As used herein, the term“substantial overpressure” refers to a pressure in excess of about 3atmospheres (about 45 psi) applied during the infiltration process,either mechanically or through the use of pressurized gas.

Upon infiltration, the molten metal enters the pore structure of theceramic filter element and fills substantially all of the void space, upto a certain height. The distance that the metal infiltrates into thefilter element can be controlled by controlling the amount of moltenmetal that is available to infiltrate or by controlling the amount oftime that the filter element is in contact with the molten metal or bycontrolling the alloy composition, and in particular the oxygen levelfor a copper alloy.

Infiltration additives can also be used when such additives are notsubstantially detrimental to the resulting seal in the filter element.For example, to improve capillary action between the ceramic and themolten metal, it may be desirable to modify the wetting or spreadingcharacteristics of the ceramic and metal. One way to do this is to coatthe open porosity of the ceramic filter element with a coating that ismore easily wetted by the molten metal. For instance, the surfaces ofalumina or magnesia ceramic can be modified by vapor phase coating theceramic with copper oxide or nickel oxide to enhance the infiltration ofcopper metal into the porous support.

Another way of enhancing the wetting characteristics is to modify thechemical composition of the molten metal with an infiltration additive.For instance, the wetting of molten copper on alumina can be enhanced bythe addition of oxygen to the copper. Such additions enhance the wettingand permit infiltration to occur rapidly and easily. When oxygen isadded, it is preferred to add the oxygen to the copper metal in anamount of from about 1.5 weight percent to about 10 weight percent, morepreferably from about 2 weight percent to about 5 weight percent.

After one or more of the surface modifications and chemical modificationnoted above, if necessary, the molten metal will wet the ceramic andinfiltrate the open porosity of the ceramic filter element throughcapillary action.

Although a vacuum atmosphere is not necessary, the metal infiltrationstep is preferably performed in a vacuum atmosphere. As used herein,vacuum atmosphere refers to an atmospheric pressure of about 10millitorr or less. The evacuation of air from the ceramic void spacereduces the likelihood that air pockets will be entrapped in thecomposite.

The temperature at which infiltration takes place is dependent on theceramic and molten metal used. In one embodiment, an alumina ceramicwith a 3 micrometer average pore size is infiltrated at 1250° C. withcopper having about 3 weight percent oxygen added as an infiltrationadditive. The total time required for infiltration of a filter elementis very short and can typically occur in less than about 1 minute.

The filter element is preferably heated to at least the meltingtemperature of the infiltrant metal prior to infiltration. If theceramic filter element is too cool, the molten metal may selectivelysolidify as it infiltrates the ceramic filter element and may preventfurther infiltration due to the solidification.

After the metal has been infiltrated into an end of the filter elementand has cooled, the filter element can then be connected to a supportingmetallic ring. For example, it is often necessary to connect the filterelement to a supporting stainless steel ring which is later connected tothe filtration apparatus.

According to one embodiment of the present invention, the metalinfiltrated filter element is brazed to a metal ring, such as astainless steel ring. The metal ring is brought into proximity with thefilter element such that the ring surrounds the ceramic-metal compositeportion and a braze metal is placed between the stainless steel ring andthe composite portion. When the braze is heated, it wets both thestainless steel ring and the ceramic-metal composite portion and, uponcooling, forms a tight hermetic bond to both articles. According to oneembodiment of the present invention, the infiltrated metal is coppermetal, the ring is stainless steel and the braze is a copper/silverbrazing alloy.

In an alternative embodiment of the present invention, a reactive brazeis infiltrated into the ceramic filter element such that the reactivebraze simultaneously adheres to the stainless steel ring. For example, acopper/silver/titanium alloy can be heated to above its melting pointand infiltrated into the ceramic while the metal ring is placed aroundthe filter element. Upon cooling, a hermetic seal is formed between themetal ring and the metal-infiltrated ceramic.

The foregoing embodiments are generally illustrated in FIGS. 2-4. FIG. 2illustrates a filter element 210 according to the present invention. Thefilter element 210 includes a plurality of filtering channels 220extending through the length of the filter element 210. FIG. 3illustrates a filter element 310 wherein the porous support portion 360has been infiltrated at the lower end 370 with a metal 380. Although, asillustrated in FIG. 3, it appears that the metal is discontinuous, themetal is in fact continuous and forms a solid surface around theperimeter of the filter element 310. However, the molten metal does notenter the channels 220, which remain free of obstruction.

FIG. 4 illustrates the filter element 410 wherein a ring 490 has beenattached to the lower end of the filter element 410. The filter element410 having the ring 490 attached can now be placed into a filtrationdevice. Although not illustrated, the ceramic filter element can have acomposite infiltrated end and a metal ring attached to both ends of thefilter element.

The sealed filter element according to the present invention provides anumber of advantages over the prior art. The metal ring is hermeticallyattached to the filter element without the need for organic o-rings orother similar techniques that may leak due to gaps in the seal. The sealaccording to the present invention will work for a wide variety offiltration devices including those devices used for separating particlesin the ultrafiltration range. Further, the operating temperature willonly be limited by the melting temperature of the metal infiltrated intothe ceramic and by the braze melting temperature, if applicable.

In another embodiment of the present invention, before, during or afterthe step of cooling the metal infiltrated filter element, a connectingmeans can be attached to the ceramic-metal composite portion of thefilter element. In one embodiment, such a connecting means permitssealable attachment of the ceramic filter element to a dissimilarmaterial. In another embodiment, such a connecting means permitssealable attachment of one ceramic filter element to another ceramicfilter element.

In general, it is desirable to use ceramic filters under extremeconditions such as aggressive chemical environments (e.g., caustic orcorrosive environments) and high temperatures, because ceramic filtersare more durable, rugged, compact and do not require replacement foryears compared to other membranes. Therefore, the ability to sealablyattach a ceramic filter of the present invention (i.e., a ceramic filterhaving a ceramic-metal composite portion) to a dissimilar material, orto another ceramic filter at the ceramic-metal composite portion isdesirable because such filters can be used in applications which involvesuch extreme conditions.

High temperature and aggressive chemical environments are typicallyproblematic when using non-metal infiltrated ceramic filters. Forexample, filtration under high temperatures can be problematic due tounequal thermal expansion between the ceramic filter element and themanifold (e.g., typically metal) to which the filter is attached.Unequal, or mismatched, thermal expansion can result in cracking of theceramic filter. Another condition which is problematic when usingnon-metal infiltrated ceramic filters is filtration of corrosive orcaustic materials, wherein the filter seals may breach due to contactwith the corrosive or caustic material. The ceramic filter element ofthe present invention overcomes these problems because the connectingmeans can be a metal that is resistant to problems associated with hightemperature and aggressive chemicals. The connecting means sealablyattaches the filter to a dissimilar material or to another filter at theceramic-metal composite portion of the filter. The infiltrated metalstrengthens the connection and improves the ability of the connection towithstand thermal expansion and/or the filtration of caustic materials.In addition, because of the strength and compliance of the ceramic-metalcomposite portion of the filter, a variety of connecting means can beused with a metal-infiltrated ceramic filter of the present inventionthat might cause a non-infiltrated ceramic to crack or otherwise fail.

According to the present invention, a connecting means can include anysuitable means for attaching a ceramic filter element to a dissimilarmaterial, or to another ceramic filter element. Preferred connectingmeans include, but are not limited to, a swage fitting, a compressionfitting, a weld, a braze, a metal bellows and/or metal threads which areformed on the ceramic metal composite portion of the filter andcombinations thereof. Several of these connecting means are discussed indetail below.

The connecting means can attach a ceramic filter element to a dissimilarmaterial or to another ceramic filter element in a variety ofconfigurations. For example, in one embodiment, illustrated in FIG. 5A,several ceramic filter elements 510 can be attached to a tube sheetheader 520 of a filtration device. In this embodiment, the ceramic-metalcomposite portions 530 of a plurality of metal-infiltrated ceramicfilter elements 510, each having a single channel, are attached to atube sheet header 520 by a connecting means (not shown in this figure)as discussed below.

In another embodiment, illustrated in FIG. 5B, a plurality of ceramicfilter elements 510 are connected to a header 520 as shown in FIG. 5A,and the entire unit is contained within a housing 540. In thisembodiment, the housing 540 is connected at the top to an inlet (notshown) and at the bottom to an outlet (not shown). The filtrant entersfrom the inlet, passes into the channels 570 of the ceramic filters,passes through the channels such that the permeate filters out of theopen porosity of the ceramic and the concentrate exits the ceramicfilter into the outlet at the bottom of the housing.

Alternatively, the bottom of a ceramic filter can be sealed by a cap1180, as illustrated in FIG. 11, so that the entire filtrant exits thefilter as a permeate. A cap can be connected to the ceramic-metalcomposite portion of a ceramic filter, for example, by welding orbrazing the cap onto the metal infiltrated end.

In another embodiment, the ceramic-metal composite portion of ametal-infiltrated ceramic filter element has a plurality of channelsthat run through the length of the filter element and are substantiallyparallel to the longitudinal axis of the filter and to one another. Thefilter is attached to a dissimilar material or to another ceramic filterelement by a connecting means as described herein.

In yet another embodiment, the ceramic-metal composite portion of twosingle channel, metal-infiltrated ceramic filter units are attached toeach other (e.g., end to end) by a connecting means. The point ofattachment between the two filters can form a joint. A joint can be anytype of joint, including a butt joint or a lap joint. Connecting meanswhich can be used to form these particular joints are described indetail below.

In one embodiment, a connecting means suitable for use in the presentinvention comprises threads. As illustrated in FIG. 6, the threads 610are typically machined into the ceramic-metal composite portion 620 ofthe ceramic filter element 630 so that the ceramic filter element can bethreadably connected (e.g., screwed) into another threaded member 640.One advantage of the infiltrated ceramic filter of the present inventionis that the ceramic-metal composite portion behaves like a metal in somerespects, allowing the machining of threads into the composite.Alternatively, the threads can be formed on top of the compositeportion. In one embodiment of this aspect of the present invention, theceramic-metal composite portion of the ceramic filter is at least about25% by weight metal. Alternatively, the threads may be formed into thegreen ceramic body prior to firing or machined into the green ceramicbody or the sintered ceramic body, all prior to metal infiltration.

In another embodiment of the present invention, a connection means canbe a swage fitting. Swage fittings may not be suitable for use withnon-metal-infiltrated ceramics, because the pressure created by applyingsuch fittings may cause the ceramic to crack or otherwise fail. This isbecause ceramics typically have little or no compliance. The metalinfiltrated ceramic of the present invention, however, is well suited tothe use of a swage fitting, because the ceramic-metal composite portionhas suitable compliance to accept the pressure applied by a swagefitting. In other words, the ceramic-metal portion has both the strengthand the compliance to accept the pressure applied by the fitting withoutcracking or breaking. As illustrated in FIG. 7, in this embodiment, thecompression ring 710 of a swage fitting is placed around theceramic-metal composite portion 720 of a metal infiltrated ceramicfilter 730. A male bolt portion 740 of the swage fitting is thenthreadably connected to a female nut portion 750 of the swage fittingover the compression ring 710 such that the compression ring 710 iscompressed onto the ceramic-metal composite portion 720, the entireswage fitting forming a tight seal with the filter. The swage fittingcan then be attached to another swage fitting which is connected to asecond ceramic filter, for example, or to another component, such as atube. A swage fitting can be made of brass or stainless steel, althoughother materials may be suitable.

In another embodiment of the present invention, a connecting means canbe a braze. Examples of brazing and materials suitable therefor havebeen previously described herein. Brazing can be used to attach a metalmember or another ceramic filter to the ceramic-metal composite portionof a ceramic filter element. For example, as described above, a metalseal ring can be attached to the perimeter of the ceramic-metalcomposite portion of a filter. The step of brazing can be performedusing any suitable brazing metal (e.g., a high-temperature solder).Preferred brazing metals include, but are not limited to, acopper-silver eutectic brazing alloy or a silver alloy. In oneembodiment, a ceramic filter element to be brazed is infiltrated withcopper, and then plated with nickel prior to the brazing process.

In another embodiment of the present invention, a connecting means caninclude brazing a ring onto the ceramic-metal composite portion of theend of a ceramic filter element. Such a ring can be used to connect orseal the ceramic filter to another component. For example, asillustrated in FIG. 8, the end of a metal-infiltrated ceramic filter 810having a ring 820 brazed thereon can be inserted into a header 830. Thering 820 can then be expanded (e.g., flared) into the header 830 to forma wedge shape that sealably connects the ceramic filter to the header.This can be accomplished, for example, by using a tapered mandril toshape the ring (e.g., hydroexpansion). The ring can be made of anysuitable metal, preferably from Kovar™, carbon steel, or stainlesssteel.

In another embodiment, the connecting means is a weld. According to thepresent invention, a weld is a fusing together of metal to form a joint.The weld can attach the ceramic-metal portion to a metal member. Such ametal member can be any member (e.g., a manifold) which is formedsubstantially of metal. Alternatively, a weld can attach theceramic-metal portion of one ceramic filter element to the ceramic-metalportion of another ceramic filter element. As illustrated in FIG. 9A,the ends of two filter elements 910 can be welded flush together, forexample, to form a butt joint 920. The joint can optionally be furtherstrengthened by brazing a ring 930 around the circumference of thefilters at the joint. It is to be understood that a butt joint can alsobe formed by brazing, instead of welding, the metal infiltrated ends oftwo filters together. The ceramic-metal composite portion of the afilter element can also be welded to another filter element by formationof a lap joint between the ceramic-metal portions of the filters. Asillustrated in FIG. 9B, such a joint can be formed, for example, byforming a tapered fit between two filter elements. In this embodiment,one metal infiltrated end 940 of a first ceramic filter 950 is taperedto fit inside the channel 960 at the metal infiltrated end of a secondinfiltrated ceramic filter 970. The first filter 950 is compressedinside the second filter 970 until a tight fit is achieved. As for abutt joint, the lap joint 980 formed by the two filters can be furtherstrengthened, if desired, by brazing a ring 930 around the circumferenceof the filters at the lap joint 980. Alternatively, a lap joint can beformed between the ceramic-metal composite ends of two metal infiltratedceramic filter units by press fitting the two filters together (notshown). In this embodiment, the external diameter of one filter is theessentially the same as the internal diameter of a second filter, andthe end of the first filter is forced inside the channel of the secondfilter to form a lap joint.

Another type of connecting means of the present invention is anexpansion bellows, or metal bellows. A metal bellows is particularlyuseful for ceramic filters which are to be used under extreme thermalconditions. In this situation, metal and ceramic have mismatched thermalexpansion properties, and it is desirable to provide an expandableconnecting means which can expand to avoid cracking or breaching at theceramic joint. FIG. 10A illustrates a metal bellows which is used as aconnecting means between two infiltrated ceramic filters of the presentinvention. In this embodiment, an end 1015 of a metal bellows 1010 canbe attached to a ceramic filter 1020 by welding or brazing the end ofthe metal bellows 1010 to the ceramic-metal composite portion 1030 ofthe end of the filter. The other end of the metal bellows can beattached similarly to the ceramic-metal composite end of another ceramicfilter. The central portion of the metal bellows 1016, which essentiallyforms the joint between the filters, is formed of expandable folds ofmetal that will allow the joint to expand under thermal conditionswithout damaging the connection between the ceramics and/or the ceramicand the metal bellows. In one embodiment of the present inventionillustrated in FIG. 10B, an infiltrated ceramic filter 1020 can beattached by its ceramic-metal composite portion 1030 to a dissimilarmaterial such as a header 1040 by a metal bellows connection means 1010.As illustrated in FIG. 10C, two or more filters 1020 can be connectedend to end using metal bellows 1010, providing a strong, yet flexiblefilter unit that can withstand extreme thermal conditions. Finally, ifthe ceramic filter is contained within a metal housing, as illustratedin FIG. 5B, a portion of the housing can include a bellows portion (notshown) which extends around the circumference of the housing and allowsthe housing to expand and contract as necessary to accommodate thethermal expansion differences between the ceramic filter and the metalhousing.

EXAMPLE

A porous support for a ceramic filter element having an apparentporosity of about 40 volume percent and a pore diameter range of fromabout 6 micrometers to about 10 micrometers is provided. The poroussupport is alumina and the channels are coated with an alumina membranefor separation of particles in the 0.2 to 0.8 micrometer size range. Thefilter element is cylindrical with 12 channels running the length of thecylinder. The cylinder has a diameter of about 1 inch (2.5 centimeters)and a length of about 39 inches (100 centimeters).

A molten metal bath is prepared for infiltrating into the filterelement. The metal comprises copper metal and includes 3 weight percentcopper oxide as an infiltration enhancer. The metal is heated to about1275° C. until the metal is molten. While under a vacuum, the filterelement is heated to about 1275° C. and one end of the filter element iscontacted with the metal bath for less than one minute and the copperinfiltrates that end of the filter to a height of about 1 inch (2.5centimeters).

The filter element is then cooled to solidify the copper metal and sealthe end of the filter element. Thereafter, a stainless steel ring isbrazed onto the end of the filter using a copper/silver braze havingabout 30 weight percent silver.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. For example,although porous ceramic filter elements have been described extensivelythroughout the specification, it is to be understood that other porousceramics can be substituted therefor. It is to be expressly understoodthat such modifications and adaptations are within the spirit and scopeof the present invention.

What is claimed is:
 1. A method for sealing an end of a ceramic filterelement, comprising the steps of: (a) providing a porous ceramic filterelement comprising a first end portion and a second end portion andcomprising filtering channels therethrough wherein said filter elementcomprises a substantially continuous network of open porosity; (b)contacting a portion of said first end of said filter element with amolten metal comprising from about 1.5 weight percent to about 10 weightpercent oxygen, to infiltrate said metal into the continuous network ofopen porosity in said first end portion of said filter element bycapillary action without substantially infiltrating said filteringchannels; (c) cooling said infiltrated portion to form a filter elementhaving an internally sealed end comprising a ceramic-metal compositeportion.
 2. The method of claim 1, wherein said molten metal comprisescopper metal.
 3. The method of claim 1, wherein said ceramic filterelement comprises a porous ceramic support having from about 30 volumepercent to about 50 volume percent open porosity.
 4. The method of claim1, further comprising the step of coating at least a portion of saidfiltering channels with a membrane layer.
 5. The method of claim 1,further comprising forming threads into said ceramic metal compositeportion before or after metal infiltration.
 6. The method of claim 5,wherein said membrane layer comprises three different layers.
 7. Themethod of claim 1, further comprising attaching a connecting means tosaid ceramic-metal composite portion.
 8. The method of claim 7, whereinsaid connecting means permits sealable attachment of said ceramic filterelement to a dissimilar material.
 9. The method of claim 7, wherein saidconnecting means permits sealable attachment of said ceramic filterelement to another ceramic filter element.
 10. The method of claim 7,wherein said connecting means comprises a swage fitting or a compressionfitting.
 11. The method of claim 7, wherein said connecting means is abraze.
 12. The method of claim 7, wherein said connecting meanscomprises a metal bellows.
 13. The method of claim 7, wherein saidconnecting means is a weld.
 14. The method of claim 13, wherein saidweld attaches said ceramic-metal composite portion to a metal member.15. The method of claim 7, wherein said step of attaching saidconnecting means to said ceramic-metal composite portion comprises thestep of brazing said connecting means to said ceramic-metal compositeportion.
 16. The method of claim 15, wherein said step of brazing saidconnecting means to said ceramic-metal composite is performed using acopper-silver brazing alloy.
 17. The method of claim 7, wherein saidconnecting means comprises threads.
 18. The method of claim 17, whereinsaid porous ceramic support has an average pore size of from about 0.01micrometers to about 2 millimeters.